A thermodynamic model is derived which is suitable to describe adsorption from a mixed protein/surfactant solution. The comparison with experimental data for HSA mixed with the nonionic surfactant decyl dimethyl phosphine oxide shows good agreement. Some model calculations are discussed in terms of the competitive character of the process of adsorption from mixed protein/surfactant solutions. The behavior of globular (HSA) and flexible (beta-casein) proteins appears to be quite different due to the possibility of changing the molar area of adsorbed protein molecules. (C) 2004 Elsevier Inc. All rights reserved.